The present invention relates in general to graphic presentation of bitmap images. It relates in particular to forming a graphic representation of a bitmap image on a vector graphics device such as a vector display or a pen plotter.
A computer-generated graphic is made up of a set of discrete color values or discrete shades of gray. These discrete values are generally referred to as picture elements or pixels. The picture elements are arranged in a regular rectangular matrix, forming in effect a raster grid. Such a matrix is termed a pixel map or pixmap.
Each picture element is represented by a binary code including one or more bits. In a high-resolution color pixmap, the picture elements may be represented, for example, by twenty-four bits, eight bits for each of three additive primary colors, red, green and blue.
A pixmap of a graphic is easily displayed on a computer CRT display as the CRT is raster-oriented. However, a pixmap can not be displayed on a vector-oriented device such as a vector display or a pen plotter; such devices are not designed to respond to signals in pixmap form.
Attempts have been made to output raster-oriented data on a pen plotter by converting a pixmap into a bitmap. In a bitmap each element is represented by only one bit of information. The bit can have either of two binary states, for example, black or white. The black elements in the bitmap are then drawn, one by one, on the plotter.
The above-described method of bitmap plotting produces poor results for several reasons. For example, while a pen plotter has an addressable resolution which may be as high as two-thousand dots per inch (2000 dpi), the effective resolution of the plotter is constrained by the width of a line drawn by a plotter pen. In most pen plotters the effective resolution is about 72 dpi or less. Accordingly, it is difficult to produce an acceptable halftone graphic, on a pen plotter, directly from a graphic bitmap.
When a bitmap graphic is plotted on a pen plotter, the plotter draws a short line representing each black element or dot in the bitmap. This may be a very slow process, since a graphic may contain over one-hundred-fifty-thousand black dots. Further, drawing such short lines causes the plotter to operate with a pecking motion. The pecking motion accelerates wear on the plotter pen to the point where dot size may be considerably larger at the end of the plot than it was at the beginning.
In order to plot a colored graphic, the graphic must be plotted at least three times, once from each of three separate bitmaps, each bitmap representing one of three subtractive primary color components of the graphic, i.e., cyan, magenta and yellow (CMY). Because of this, the above-described problems of plotting a monochrome bitmap are greatly magnified.